1. Field of the Invention
This invention relates generally to digital video compression and, in particular, to encoding a previously encoded digital video signal, also known as transcoding.
2. Description of the Related Art
Consumer electronic equipment increasingly uses digital video technology. Because it improves picture quality, this digital technology is superior to the analog technology currently used in most commercial broadcasting and traditional VCRs. With digital video, motion picture image information is stored in the form of bits of digital data, i.e. 0s and 1s. This information may be transmitted in the form of a stream of bits, also known as a xe2x80x9cdigital video signal.xe2x80x9d Conventional digital video signals, however, require undesirably wide channels for transmission and undesirably large amounts of memory for storage. To avoid these problems, digital video signals are often xe2x80x9ccompressedxe2x80x9d or xe2x80x9cencoded.xe2x80x9d Compression and encoding allow the same video information (or nearly the same information) to be represented using fewer bits. These encoding techniques have allowed for technical advances in the fields of digital broadcast television, digital satellite television, video teleconferencing, and video electronic mail.
Digital video compression reduces the number of bits by removing redundant information, without seriously affecting the quality of the video. Standard video compression techniques remove spatial redundancy within a video frame and remove temporal redundancy between video frames. To remove spatial redundancy, encoders commonly use a Discrete Cosine Transform (DCT), which is widely known and understood. To remove temporal redundancy, encoders commonly use motion estimation, which is also widely known and understood.
With regard to motion estimation, the images in a digital video usually do not change much within small time intervals, i.e., adjacent frames include a great deal of redundant information. Thus, motion-estimation takes advantage of this redundancy and encodes a video frame based on other video frames temporally close to it. For example, in a particular movie scene, the background trees (outdoor scene) or furniture (indoor scene) may not move. Therefore, video information related to the background may not necessarily have to be transmitted multiple times, reducing the number of bits to be transmitted or stored. On the other hand, if the camera is panning, the background may xe2x80x9cmovexe2x80x9d on the video screen. In this case, it is possible to avoid transmitting background information. Instead of encoding and transmitting the background information multiple times, it is possible to encode it once and subsequently transmit information related to its movement. Techniques related to this process are called motion estimation.
A device known as a transcoder may employ motion estimation. A transcoder reduces the bit-rate of an already compressed video bit-stream, allowing the bit stream to be transmitted through a narrower channel. Transcoders are often used by video services that operate over more than one type of network. In this situation, the different networks may have different bandwidths, thus each end-user may require a different Quality of Service (QoS). Therefore, xe2x80x9cgatewaysxe2x80x9d between the networks employ transcoders to adapt video bit-rates to different end-users on different networks.
There are two well known types of transcoders. First, the simplest type is an xe2x80x9copen-loop transcoder.xe2x80x9d In this type of transcoder, the incoming bit-rate is reduced by the well-known mathematical technique of truncating or re-quantizing the DCT coefficients. In other words, the encoded bits that represent the higher quality aspects of the video are discarded. Because this transcoding is done in the xe2x80x9ccoded domain,xe2x80x9d i.e., it is done without decoding the signal, these transcoders are simple and fast. Open-loop transcoding, however, produces increased distortion caused by a xe2x80x9cdriftxe2x80x9d due to the mismatched reconstructed pictures in the encoder and the decoder. This distortion may result in an unacceptable video quality in many applications.
A second, more complicated, type of transcoder is a xe2x80x9cdrift-freexe2x80x9d transcoder. It operates by decoding the incoming coded video and then re-encoding the video at a lower bit-rate. Using this method, it is possible to take advantage of useful information in the encoded video arriving at the transcoder, such as picture type, motion vectors, quantization step-size, bit-allocation statistics, etc. It is possible to construct transcoders with different complexity and performance with regard to coding efficiency and video quality.
Traditionally, the second type of transcoding does not employ motion estimation for two reasons. First, transcoders must operate very quickly and motion estimation is computationally complex and thus expensive to implement. Second, it is widely assumed that re-using motion vectors extracted from the incoming encoded video is as good as performing a new motion estimation, thus motion estimation is not performed in the transcoder. In some applications, however, this reuse scheme introduces significant quality degradation because reused motion vectors are not the optimal motion vectors.
Thus, it is thus desirable to provide a method and apparatus of motion estimation when re-encoding a digital video signal that is not computationally complex, thereby providing an improved video quality.
The advantages and purposes of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention are realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method and apparatus of re-encoding digital video from a previously encoded digital video having at least one input motion vector. The method comprises the steps of determining a base motion vector from the at least one input motion vector, generating a delta motion vector, generating a new motion vector that is the sum of the base motion vector and the delta motion vector, and re-encoding the previously encoded digital video using the new motion vector.
The summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show several embodiments of the invention and, together with the description, explain the principles of the invention.